1. Field of the Invention
The present invention relates to a color copier, color laser printer, color facsimile apparatus or similar color image forming apparatus.
2. Description of the Background Art
A tandem, color image forming apparatus, belonging to a family of color image forming apparatuses, includes four photoconductive drums or image carriers arranged side by side. While a belt is conveying a sheet or recording medium via the drums, toner images of different colors formed on the drums are sequentially transferred to the sheet one above the other by biases applied to bias applying members, completing a color toner image on the sheet. In such an apparatus, the belt conveys the sheet while electrostatically retaining it thereon, so that the surface speed or moving speed of the sheet is equal or substantially equal to the surface speed of the belt.
If the surface speed of the belt and that of the drums are the same as each other, then image transfer is effected only by an electrostatic force. On the other hand, when the surface speed of the belt is made different from the surface speed of the drums, a mechanical peeling force acts in addition to the electrostatic force and obviates defective image transfer. More specifically, when a difference exists between the two surface speeds, desirable image transfer is attainable particularly when toner images of two or more colors are superposed. As far as a monochromatic image is concerned, a toner layer can be easily retained on the surface of a sheet, so that transferability above a certain level is easily achievable. However, when two or more colors are superposed, a toner layer previously transferred to a sheet lowers the transferability of the next toner layer. This problem can be effectively coped with if a difference is provided between the surface speed of the belt and that of the drums.
It is a common practice with a color image forming apparatus configured to enhance transferability of two or more colors to provide a difference between the surface speed of the belt and that of the drums and maintain the difference constant. Stated another way, the above difference is generally not expected to be varied by the user of the apparatus or a service person.
It has been customary with an electrophotographic, color image forming apparatus to sequentially transfer toner images of different colors to a single sheet one above the other for thereby forming a color image. Regarding this kind of apparatus, Japanese Patent No. 2,743,359 discloses an image transferring device capable of preventing an image from being distorted at the time of transfer. The image transferring device taught in this document allows toner images of different colors to be transferred to a sheet in accurate register without resorting to high dimensional accuracy even when a plurality of drums are used. More specifically, the image transferring device is applied to a tandem, color image forming apparatus in which toner images are sequentially transferred from a plurality of drums to a sheet being carried by an image transfer body before the image transfer body completes one rotation. The surface speed of the image transfer body is made higher than the surface speed of the drums by 0.1% to 1%. A flexible member is fitted on the circumference of the image transfer body and elastically pressed against the drums.
However, the Japanese Patent mentioned above simply teaches a method capable of maintaining the speed of a sheet constant without regard to the extension or the positional shift of an image that may occur due to a difference in diameter between the drums or the eccentricity of the drums. Further, the above document does not show or describe a method of varying a speed ratio in accordance with the mode. In this connection, a method of enhancing transferability by providing a difference between the speed of an image transfer belt, which bifunctions as a conveyor, and the speed of the drums is conventional with a monochromatic copier.
Japanese Patent Laid-Open Publication No. 11-52794 relates to an image forming apparatus of the type providing a difference between the peripheral speed of drums and that of an endless image transfer belt, e.g., an intermediate image transfer belt. This document contemplates to obviate color shift, color change and other defects ascribable to the relative position of the drums and belt that varies color by color, thereby stably producing high-quality color images.
More specifically, in the Laid-Open Publication mentioned above, the belt is provided with a circumferential length which is non-integral times as great as the circumference of the individual drum. Further, assume that each drum has a circumference of Ld and moves at a peripheral speed of Vd, that the belt has a circumferential length of Lb and moves at a peripheral speed of Vb, that a speed difference ratio of the belt to the drum is ΔV (≠0), and that n is an integer. Then, the above document defines the relation between the circumferential length of the belt and the circumference of the drum as:Vb=Vd×(1+ΔV)Lb=Ld×(1+ΔV)×n
With this scheme, however, it is difficult to surely reduce color shift when the speed of a sheet minutely varies due to the influence of the difference in speed between the belt and the drums, which is ascribable to a change in the kind of a sheet or the variation of temperature or that of humidity. Moreover, the above document does not teach a method of varying the speed ratio in accordance with the mode.
As stated above, by providing a difference in speed between the belt and the drums, it is possible to obviate a vermicular image, e.g., characters blank inside and to enhance transferability of two or more colors. However, although a sheet is usually expected to electrostatically adhere to the belt, the above difference is apt to reduce the adhesion and thereby make the conveying speed of the sheet from coinciding with the conveying speed of the belt, bringing about color shift in the subscanning direction. More specifically, the adhesion of the sheet to the belt is dependent on the kind of a sheet and humidity, so that optimum conditions, sufficiently taking account of humidity and the kind of a sheet, must be set and maintained in order to obviate such color shift.